Multi-layered zinc alloy plated steel having excellent spot weldability and corrosion resistance

ABSTRACT

Provided is a multilayer zinc alloy plated steel material comprising a base steel material and multiple plating layers formed on the base steel material, wherein each of the multiple plating layers includes one of a Zn plating layer, a Mg plating layer, and a Zn—Mg alloy plating layer, and the ratio of the weight of Mg contained in the multiple plating layers to the total weight of the multiple plating layers is from 0.13 to 0.24.

TECHNICAL FIELD

The present disclosure relates to a multilayer zinc alloy plated steelmaterial having excellent spot weldability and corrosion resistance, andmore particularly, to a multilayer zinc alloy plated steel materialhaving excellent spot weldability and corrosion resistance and (being)applicable to automobiles, home appliances, and construction, etc.

BACKGROUND ART

Zinc plating methods for suppressing the corrosion of iron by cathodicprotection provide high corrosion resistance and economical efficiencyand are thus widely used for manufacturing steel materials having highcorrosion resistance, and there has been increasing demand for zincplated steel materials in industrial fields such as automobiles, homeappliances, constructions.

When a zinc plated steel material is exposed to a corrosive environment,zinc having a lower oxidation-reduction potential than iron undergoescorrosion first, and thus corrosion of the steel material is suppressedby sacrificial corrosion protection. Along with this, dense corrosionproducts are formed on the surface of the steel material while zinc ofthe plating layer is oxidized, thereby protecting the steel materialfrom the corrosive environment and improving the corrosion resistance ofthe steel material.

However, air pollution and corrosive environments have increased withindustrial advances, and regulations on resources and energy savingshave been tightened. Therefore, the need to develop steel materialshaving higher corrosion resistance than existing zinc plated steelmaterials has increased. To this end, various studies have beenconducted on the technology for manufacturing zinc alloy plated steelmaterials which have improved corrosion resistance by the addition ofelements such as magnesium (Mg) to plating layers.

In general, zinc plated steel materials or zinc alloy plated steelmaterials (hereinafter, referred to as “zinc plated steel materials”)are used as products after being processed into parts through formingprocess etc. and being welded through welding processes such as a spotwelding process. In the case of zinc plated steel materials having basesteel materials such as high-strength steel materials having austeniteor retained austenite as a microstructure or high-strength interstitialfree (IF) steel materials having a high phosphorus (P) content, moltenzinc permeate into the base steel materials along grain boundariesduring a spot welding process, causing brittle cracks, that is, liquidmetal embrittlement (LME).

FIG. 1 is an enlarged image showing a weld zone of welded members inwhich LME cracking occurred as a result of spot welding. In FIG. 1 ,cracks formed in the upper and lower portions of a nugget are referredto as type A cracks, a crack formed in a weld shoulder portion isreferred to a type B cracks, and a cracks formed inside a steel sheetbecause of misalignment of electrodes during welding is referred to as atype C cracks. Among such cracks, type B and C cracks greatly affect thesoundness of materials, and it is a key requirement in the art toprevent the formation of cracks during welding.

DISCLOSURE Technical Problem

An aspect of the present disclosure may provide a multilayer zinc alloyplated steel material having high spot weldability and corrosionresistance.

Technical Solution

According to an aspect of the present disclosure, a multilayer zincalloy plated steel material may include a base steel material andmultiple plating layers formed on the base steel material, wherein eachof the multiple plating layers may include one of a Zn plating layer, aMg plating layer, and a Zn—Mg alloy plating layer, and the ratio of theweight of Mg contained in the multiple plating layers to the totalweight of the multiple plating layers may be from 0.13 to 0.24.

Advantageous Effects

One of the effects of the present disclosure is that the multilayer zincalloy plated steel material has high spot weldability, and thus evenwhen a steel material such as a high-strength steel material includingaustenite or retained austenite as a microstructure, or a high-strengthinterstitial free (IF) steel material having a high phosphorus (P)content is used as a base steel material, the occurrence of liquid metalembrittlement (LME) may be effectively suppressed.

In addition, the multilayer zinc alloy plated steel material of thepresent disclosure may have high corrosion resistance even though theplating amount thereof is low, and thus the multilayer zinc alloy platedsteel material may be environmentally friendly and economical.

In addition, according to embodiments of the present disclosure, themultilayer zinc alloy plated steel material may have high platability.

In addition, according to embodiments of the present disclosure, themultilayer zinc alloy plated steel material may have improved phosphatetreatment properties.

Aspects and effects of the present disclosure are not limited thereto,and may be apparently understood through the descriptions of embodimentsof the present disclosure.

DESCRIPTION OF DRAWINGS

FIG. 1 is an enlarged image showing a weld zone of welded members inwhich LME cracking occurred as a result of spot welding.

FIG. 2 is a phase diagram of a Mg—Zn binary alloy system.

FIG. 3 is schematic views showing the corrosion processes of platedsteel materials.

FIG. 4 is a schematic diagram illustrating a multilayer zinc alloyplated steel material 100 according to an embodiment of the presentdisclosure.

FIG. 5 is a schematic diagram illustrating a multilayer zinc alloyplated steel material 200 according to another embodiment of the presentdisclosure.

FIG. 6 is a schematic diagram illustrating a multilayer zinc alloyplated steel material 300 according to another embodiment of the presentdisclosure.

FIG. 7 is a schematic diagram illustrating an electromagnetic levitationphysical vapor deposition apparatus.

FIG. 8 is an image showing a weld zone formed by a spot welding in amultilayer zinc alloy plated steel material of Inventive Sample 18.

BEST MODE

It is known that as the content of Mg increases in Zn—Mg alloy platedsteels, the Zn—Mg alloy plated steels increase in corrosion resistancebut decrease in spot weldability, and thus the maximum content of Mg inplating layers is generally adjusted to be about 10 wt %. The reason forthis is that Zn—Mg intermetallic compounds having a low melting pointeasily dissolve in a Zn—Mg plating layer to cause liquid metalembrittlement. However, additional studies of the present inventors haveshowed that even if the content of Mg in a plating layer exceeds 10 wt%, spot weldability is markedly improved in a certain Mg content range.In particular, this spot weldability improving effect is obtainable inthe case of forming two or more plating layers as well as in the case offorming a single plating layer, and based on this, the present inventorshave found that additional effects such as improvements in platabilityand phosphate treatment properties are also obtainable and have inventedthe present invention.

Hereinafter, a zinc alloy plated steel material having high spotweldability and corrosion resistance will be described in detail. In thepresent disclosure, the upper and lower sides of a steel sheet may bereversed at any time depending on the state of stacking, and thus theexpression “on” such as “on a base steel material” merely refers to thestate of contact but does not refers to or imply the state of beinglocated on an upper side.”

The zinc alloy plated steel material of the present disclosure includesa base steel material and multiple plating layers formed on the basesteel material. In the present disclosure, the base steel material isnot limited to particular types. For example, the base steel materialmay be a steel sheet or a steel wire rod.

Furthermore, in the present disclosure, the composition of the basesteel material is not particularly limited. For example, the base steelmay include, by wt %, C: 0.10% to 1.0%, Si: 0.5% to 3%, Mn: 1.0% to 25%,Al: 0.01% to 10%, P: 0.1% or less (excluding 0%), S: 0.01% or less(excluding 0%), and the balance of Fe and inevitable impurities, and thecontents of C, Si, Mn, P, and S contained in the base steel material maysatisfy Condition 1 below. In addition, the base steel material havingthe above-described composition may include austenite or retainedaustenite as a microstructure.[C]+[Mn]/20+[Si]/30+2[P]+4[S]≥≥0.3  Condition 1:

(where each of [C], [Mn], [Si], [P], and [S] refers to the content (wt%) of a corresponding element)

In the case that the base steel has above-described composition andmicrostructure, liquid metal embrittlement (LME) may be a major problemfor the following reasons. That is, austenite or retained austenite hasweaker grain boundaries than other phases, and thus when stress isapplied thereto during spot welding, molten zinc may penetrate into thegrain boundaries of austenite or retained austenite in a weld zone tocause cracks, that is, brittle fracture by liquid metal embrittlement(LME).

However, according to the present disclosure, as described later, thetime during which molten zinc remains is minimized, and thus theoccurrence of liquid metal embrittlement (LME) may be effectivelysuppressed even if a zinc alloy plated steel material is manufacturedusing a steel material having the above-described composition andmicrostructure as a base steel material. In addition, the idea of thepresent disclosure may even be applied to a base steel material thatdoes not satisfy the above-described composition.

Each of the multiple plating layers is one of a Zn plating layer, a Mgplating layer, and a Zn—Mg alloy plating layer, and the presentdisclosure has a technical feature in which the ratio of the weight ofMg contained in the multiple plating layers to the total weight of themultiple plating layers is from 0.13 to 0.24. More preferably, the Mgweight ratio may be within the range of 0.157 to 0.20.

The Zn—Mg alloy plating layer may have a microstructure comprising a Znsingle phase, a Mg single phase, a Mg₂Zn₁₁ alloy phase, a MgZn₂ alloyphase, a MgZn alloy phase, a Mg₇Zn₃ alloy phase, or the like, and thepresent inventors have found that if the content of Mg contained in themultiple plating layers is adjusted within the above-mentioned range,the multiple plating layers are melted and changed to a single alloylayer including a MgZn₂ alloy phase in an area fraction of 90% orgreater (including 100%) in a weld zone during spot welding, and thusliquid metal embrittlement (LME) is effectively suppressed. The reasonfor this may be that, as shown in the phase diagram of a Mg—Zn binaryalloy system in FIG. 2 , the melting point of the single alloy layer ishigh, and thus the time during which the single alloy layer remains in aliquid state is minimized. Furthermore, in the present disclosure, othermicrostructures than the MgZn₂ alloy phase in the single alloy layer arenot particularly limited. In a non-limiting example, the single alloylayer may include a Mg₂Zn_(n) alloy phase as a remainder in addition tothe MgZn₂ alloy phase.

Here, phase fractions may be measured and analyzed by a standardRietveld quantitative analysis method using an XRD and a more accurateTEM-based crystal orientation mapping technique (TEM-ASTAR) as well, butthe present disclosure is not limited thereto. In addition, the phasetransformation process of the Zn—Mg alloy plating layer may be analyzedusing high-temperature in-situ synchrotron XRD. More specifically, thephase transformation process of the Zn—Mg alloy plating layer may beanalyzed by heating specimens to 780° C. at heating rates of 1.3° C./secand 11.3° C./sec and measuring XRD spectra at a frame rate of 1 second,totally consecutive 900 frames during a heating and cooling thermalcycle. However, this is a non-limiting example.

According to an additional study conducted by the present inventors,although the content of Mg is adjusted to be within the above-describedrange, it may be difficult to improve spot weldability if the content ofMg excessively deviates in the width direction of the plating layers (adirection perpendicular to the direction of rolling). Thus, it may berequired to appropriately control the upper limit of the deviation ofthe Mg content in the width direction of the multiple plating layers,and preferably, the deviation of the Mg content may be controlled to bewithin ±5% when a GDS profile is measured in a thicknesswise centerportion of each of the multiple plating layers.

According to an additional study conducted by the present inventors, theaverage grain diameter of the multiple plating layers has a significanteffect on the corrosion resistance of the plated steel material. FIGS.3A and 3B are schematic views showing the corrosion processes of platedsteel materials. FIG. 3 (a) is a schematic view shown the case in whichgrains are relatively fine, and FIG. 3 (b) is a schematic viewillustrating the case in which grains are relatively coarse. Referringto FIGS. 3A and 3B, it could be understood that that when the size ofgrains is small, relatively dense and uniform corrosion products areformed while corrosion occurs, and thus corrosion may be delayed by thedense and uniform corrosion products.

In addition, the average grain diameter of the multiple plating layersalso has a significant effect on the spot weldability of the platedsteel material. When the average grain diameter is equal to or less thana certain value, the formation of type B cracks is remarkably reduced,and the reason for this may be that the migration of atoms in a moltenplating layer actively occurs and thus the formation of an intendedmicrostructure is facilitated.

Thus, considering the corrosion resistance and spot weldability of theplated steel, it may be required to appropriately adjust the upper limitof the average grain diameter of the multiple plating layers, and theaverage grain diameter of the multiple plating layers may be preferablyadjusted to be 100 nm or less (excluding 0 nm). Here, the average graindiameter refers to the average long diameter of grains measured byobserving thicknesswise cross-sections of the multiple plating layers.

According to an example of the present disclosure, the sum of platingamounts of the multiple plating layers may be within the range of 40g/m² or less (excluding 0 g/m²). As the sum of plating amounts of themultiple plating layers increases, corrosion resistance may increase butliquid metal embrittlement (LME) may occur during spot welding, and thusthe upper limit of the sum of plating amounts may be adjusted asdescribed above. In addition, considering both the corrosion resistanceand the spot weldability, the sum of plating amounts of the multipleplating layers may be adjusted to be preferably within the range of 10g/m² to 35 g/m², and more preferably within the range of 15 g/m² to 30g/m².

In addition, as described above, the zinc alloy plated steel material ofthe present disclosure is characterized by including two or more platinglayers to improve corrosion resistance and weldability as describedabove, and platability and phosphate treatment properties as well.Hereinafter, this will be described in detail with specific embodiments.

FIG. 4 is a schematic diagram illustrating a multilayer zinc alloyplated steel material 100 according to an embodiment of the presentdisclosure.

According to the embodiment of the present disclosure, multiple platinglayers may include a first plating layer 110 formed on a base steelmaterial and a second plating layer 120 formed on the first platinglayer 110, wherein the first plating layer 110 may include a Zn singlephase or a Zn single phase and a Zn—Mg alloy phase and may have a Mgcontent within the range of 7 wt % or less, and the second plating layer120 may include a Zn—Mg alloy phase. In this case, each of the multipleplating layers may further include an additional alloy phase in additionto the Zn single phase and the Zn—Mg alloy phase.

A Zn—Mg alloy phase such as a Mg₂Zn_(n) alloy phase, a MgZn₂ alloyphase, a MgZn alloy phase, or a Mg₇Zn₃ alloy phase is an intermetalliccompound which not only has high hardness but also high brittleness,thereby lowering platability and causing separation of a plating layerwhen a zinc alloy plated steel material is formed. Accordingly, thepresent inventors have tried to impart ductility to the first platinglayer 110 formed adjacent to the base steel material in order tocompensate for the increase in the brittleness of the plating layers dueto the formation of the Zn—Mg alloy phase, and have found, as one methodfor this, that when the first plating layer 110 is a Zn plating layer ora Zn—Mg alloy plating layer having a Mg content of wt % or less(preferably, 6.3 wt % or less, and more preferably 5.5 wt % or less),plating adhesion may be significantly improved.

For example, the first plating layer 110 may have a composite phaseincluding a Zn single phase and a Mg₂Zn_(n) alloy phase, and in thiscase, the first plating layer 110 may include the Zn single phase in anarea fraction of 20% or greater. When the first plating layer 110 hasthe above-described microstructure, the first plating layer 110 may havehigh compressive strength such that the first plating layer 110 mayabsorb and buffer stress during forming and may thus result in highplatability.

According to an example of the present disclosure, the plating amount ofthe first plating layer 110 may be 3 g/m² or greater. In the presentembodiment, platability may be sufficiently improved as intended bycontrolling the plating amount of the first plating layer 110 asdescribed above. In an embodiment of the present disclosure, the platingamount of 3 g/m² may correspond to a thickness of 0.6 μm.

FIG. 5 is a schematic diagram illustrating a multilayer zinc alloyplated steel material 200 according to another embodiment of the presentdisclosure.

In an embodiment of the present disclosure, multiple plating layers mayinclude a first plating layer 210 formed on a base steel material and asecond plating layer 220 formed on the first plating layer 210, whereinthe first plating layer 210 may include a Zn—Mg alloy phase and thesecond plating layer 220 may include a Zn single phase or a Zn singlephase and a Zn—Mg alloy phase and may have a Mg content within the rangeof 2 wt % or less. In this case, each of the multiple plating layers mayfurther include an additional alloy phase in addition to the Zn singlephase and the Zn—Mg alloy phase.

If the outermost surface of the zinc alloy plated steel material 200includes the Zn—Mg phase in a certain amount or greater, the phosphatetreatment properties of the zinc alloy plated steel material maydeteriorate. The reason for this is that the corrosion potentialdifference between the Zn—Mg alloy phase and Ni ions included in aphosphate treatment solution causes galvanic corrosion and thus promotesthe dissolution of the plating layers, and as a result, pits exposingthe base steel material are formed. Considering this, the second platinglayer 220 located on the outermost surface of the zinc alloy platedsteel material 200 may be controlled to include only the Zn singlephase, or the fraction of the Zn—Mg alloy phase of the second platinglayer 220 may be adjusted to be a certain value or less, so as toeffectively improve the phosphate treatment properties of the zinc alloyplated steel material 200.

For example, the plating amount of the second plating layer 220 may be 2g/m² or greater. In the present embodiment, phosphate treatmentproperties may be sufficiently improved as intended by controlling theplating amount of the second plating layer 220 as described above.

FIG. 6 is a schematic diagram illustrating a multilayer zinc alloyplated steel material 300 according to another embodiment of the presentdisclosure.

According to an embodiment of the present disclosure, multiple platinglayers may include first to third plating layers 310, 320, and 330sequentially formed on a base steel material, wherein the first platinglayer 310 may include a Zn single phase or a Zn single phase and a Zn—Mgalloy phase and may have a Mg content within the range of 7 wt % orless, the second plating layer 320 may include a Zn—Mg alloy phase, andthe third plating layer 330 may include a Zn single phase or a Zn singlephase and a Zn—Mg alloy phase and may have a Mg content within the rangeof 2 wt % or less. In this case, each of the multiple plating layers mayfurther include an additional alloy phase in addition to the Zn singlephase and the Zn—Mg alloy phase.

When the zinc alloy plated steel material 300 sequentially includes thefirst to third plating layers 310, 320 and 330, the corrosionresistance, the spot weldability, the platability, and the phosphatetreatment properties of the zinc alloy plated steel material 300.

The first plating layer 310 may have a composite phase including a Znsingle phase and a Mg₂Zn₁₁ alloy phase, and in this case, the firstplating layer 310 may include the Zn single phase in an area fraction of20% or greater. When the first plating layer 310 has the above-describedmicrostructure, the first plating layer 310 may have high compressivestrength such that the first plating layer 110 may absorb and bufferstress during machining and may thus result in high platability.

In this case, the plating amount of the first plating layer 310 may be 3g/m² or greater, and the plating amount of the third plating layer 330may be 2 g/m² or greater.

Except that the multiple plating layers include the first to threelayers, the zinc alloy plated steel material may have the same featuresas that of the multilayer zinc alloy plated steel materials of theprevious embodiments which are common in this embodiment.

The zinc alloy plated steel materials of the present disclosure may bemanufactured by various methods without limitations. For example,according to an embodiment, the following manufacturing method may beused.

First, a base steel material may be prepared, and a pickling processusing an aqueous solution having HCl in an amount of 14 wt % or greater,a rinsing process, and a drying process may performed on the base steelmaterial. Then, foreign substances and a natural oxide film may beremoved from the surface of the base steel material by using plasma, ionbeams, or the like. Next, the zinc alloy plated steel material can beobtained by forming multiple plating layers sequentially on the basesteel material.

At this time, each of the multiple plating layers may be formed by anelectroplating method or a general vacuum deposition method such as anelectron beam method, a sputtering method, a thermal evaporation method,an induction heating evaporation method, an ion plating method. However,a Mg plating layer or a Zn—Mg alloy plating layer may be formed by anelectromagnetic levitation physical vapor deposition method having anelectromagnetic stirring effect.

Here, the term “electromagnetic levitation physical vapor deposition”refers to a method using a phenomenon in which if electromagnetic forceis generated by applying high frequency power to a pair ofelectromagnetic coils generating an alternating electromagnetic field, acoating material (Zn, Mg or a Zn—Mg alloy in the present disclosure) islevitated in a space surround by the electromagnetic field withoutexternal help, and the levitated coating material produces a largeamount of deposition vapor (metal vapor). FIG. 7 is a schematic viewillustrating an apparatus for such electromagnetic levitation physicalvapor deposition. Referring to FIG. 7 , a large amount of depositedvapor formed by the above-described method is sprayed onto the surfaceof a base steel material through a plurality of nozzles of a vapordistribution box at high speed, thereby forming a plating layer.

In a general vacuum vapor deposition apparatus, a coating material isprovided in a crucible, and the coating material is vaporized by heatingthe crucible. In this case, it is difficult to supply sufficient thermalenergy to the coating material because of melting of the crucible andthermal loss at the crucible. This decreases the rate of deposition andlimits grain refinement of a plating layer. In addition, there is alimit to the uniformity of a plating layer when a Zn—Mg alloy vapor isdeposited as is in the present disclosure.

However, when deposition is performed by the electromagnetic levitationphysical vapor deposition method, there are no temperature limitationsunlike in a general vacuum deposition method, and thus the coatingmaterial may be exposed to a higher temperature environment. Therefore,deposition may be performed at a high rate, and thus a plating layerhaving fine grains and uniformly distributed alloying elements may beformed.

During a deposition process, the degree of vacuum inside a vacuumdeposition chamber may be preferably adjusted to be within the range of1.0×10⁻³ mbar to 1.0×10⁻⁵ mbar, and in this case, an increase inbrittleness and deterioration of material properties may be effectivelyprevented when a plating layer is formed.

In the deposition process, preferably, the temperature of the levitatedcoating material may be adjusted to be 700° C. or greater, morepreferably 800° C. or greater, and even more preferably 1000° C. orgreater. If the temperature of the levitated coating material is lessthan 700° C., grain refinement and plating layer uniformity may not besufficiently guaranteed. In addition, as the temperature of thelevitated coating material increases, intended technical effects may beeasily obtained, and thus the upper limit of the temperature of thelevitated coating material is not limited to a particular value in thepresent disclosure. However, if the temperature is higher than a certainvalue, such effects are saturated, and processing costs increaseexcessively. Thus, the upper limit of the temperature may be set to be1500° C.

Preferably, the temperature of the base steel material may be adjustedto be 100° C. or less before and after the deposition process. If thetemperature of the base steel material exceeds 100° C., the temperatureof the base steel material may not be uniform in the width directionthereof, and thus the base steel material may have a radius curve in thewidth direction thereof, making it difficult to maintain the degree ofvacuum when the base steel material passes through a multistagedifferential pressure reducing system at an exit side.

MODE FOR INVENTION

Hereinafter, the present disclosure will be described more specificallythrough examples. However, the following examples are for illustrativepurposes only and are not intended to limit the scope of the presentdisclosure. The scope of the present invention is defined by theappended claims, and modifications and variations reasonably madetherefrom.

EXAMPLES

High-strength cold rolled steel sheets for automobiles, having athickness of 1.4 mm and including, by wt %, C: 0.16%, Si: 1.43%, Mn:2.56%, Al: 0.04%, P: 0.006%, S: 0.0029%, and the balance of Fe andinevitable impurities, were prepared, and multilayer zinc alloy platedsteel materials including multiple plating layers having compositions asshown in Table 1 below were fabricated from the prepared high-strengthcold rolled steel sheets by using the apparatus shown in FIG. 7 (vacuumdegree: 3.2×10⁻³ mbar). In each example, each plating layer was formedthrough a separate process using a separate vacuum chamber, and wheneach plating layer was formed, a current of 1.2 kA was applied to a pairof electromagnetic coils, a frequency applied to the pair ofelectromagnetic coils was adjusted to be 60 kHz based on 2 kg of adeposition material, the temperature of a levitated coating material wasset to 1000° C., and the temperature of the vapor distribution box wasconstantly maintained at 900° C. In addition, the temperature of thebase steel was constantly maintained at 60° C. before and after eachplating layer was deposited.

Next, the plating amount and the Mg weight ratio of each multilayer zincalloy plated steel material were measured using an inductively coupledplasma (ICP) method. More specifically, a specimen having a size of 80mm×80 mm was cut out, the surface of the specimen was degreased, andthen, the weight of the specimen was primarily measured using ahigh-precision scale (primary weight W₁: 0.0000 g). Thereafter, thefront side of the specimen was attached to a column for an 54.5 mmdiameter O-ring by using a clamp and was tightened to prevent leakage ofa solution. Then, 30 cc of a 1:3 HCl solution was added, and two orthree drops of an inhibitor were added. After the generation of H₂ gason a surface, the solution was collected in a 100-cc mass flask. At thattime, all the remaining solution was collected using a cleaning bottlein an amount of 100 cc or less. Thereafter, the specimen was completelydried, the weight of the specimen was measured (secondary weight W₂),and the value obtained by dividing the difference between the primaryweight W₁ and the secondary weight W₂ by a unit area was taken as atotal plating amount. In addition, the content of Mg was measured usingthe collected solution by an ICP method as a Mg weight ratio.

Next, a GDS profile was measured at a thicknesswise center portion ofeach of the multiple plating layers, and the average grain diameter ofthe multiple plating layers was measured. Results of measurement showedthat the content of Mg deviated within the range of ±5% and the averageparticle diameter was 100 μm or less in all the examples.

Next, the weldability, corrosion resistance, powdering resistance, andphosphate treatment properties of each multilayer zinc alloy platedsteel material were evaluated, and Table 2 below shows results thereof.

More specifically, according to SEP 1220-2, specimens having a size of40 mm×120 mm were prepared by cutting, and the formation of type Bcracks and the size of type B cracks were measured after performing aspot welding process on each specimen 100 times. Then, weldability wasevaluated according to the following criteria.

1. Very good: no type B cracks were formed in all specimens.

2. Good: type B cracks were formed in some or all specimens, and theaverage length of the type B cracks was equal to or less than 0.1 timesthe thickness of the base steel material (cold rolled steel sheet).

3. Normal: type B cracks were formed in some or all specimens, and theaverage length of the type B cracks was greater than 0.1 times thethickness of the base steel material (cold rolled steel sheet) but equalto or less than 0.2 times the thickness of the base steel material.

4. Poor: type B cracks were formed in some or all specimens, and theaverage length of the type B cracks was greater than 0.2 times thethickness of the base steel material (cold rolled steel sheet).

Corrosion resistance was evaluated by cutting specimens having a size of75 mm×150 mm out of the multilayer zinc alloy plated steel materials,performing a salt spray test on the specimens by JIS 22371 to measuretimes of the occurrence of initial red rust, and performing evaluationaccording to the following criteria.

1. Good: the time period until the occurrence of red rust was equal toor greater than twice the red rust occurrence time period of a zincplated steel sheet (GI steel sheet) having a plating amount of 60 g/m²on one side.

2. Normal: the time period until the occurrence of red rust was similarto the red rust occurrence time period of a zinc plated steel sheet (GIsteel sheet) having the plating amount of 60 g/m² on one side, or lessthan twice the red rust occurrence time period of the zinc plated steelsheet (GI steel sheet).

3. Poor: the time period until the occurrence of red rust was less thanthe red rust occurrence time period of the zinc plated steel sheet (GIsteel sheet) having a plating amount of 60 g/m² on one side.

Powdering resistance was evaluated by cutting specimens having a size of40 mm×80 mm out of the multilayer zinc alloy plated steel materials,setting the specimens to a press tester, performing a 60° bending teston the specimens, detaching the specimens from the tester, attachingcellophane tape to bent portions of the specimens, separating thecellophane tape, attaching the separated cellophane tape to white paperto measure peeled-off widths, and performing evaluation according to thefollowing criteria.

1. Good: peeled-off width was 6.0 mm or less.

2. Normal: peeled-off width was greater than 6.0 mm but equal to or less8.0 mm.

3. Poor: peeled-off width was greater than 8.0 mm.

Phosphate treatment properties were evaluated by cutting specimenshaving a size of 75 mm×150 mm out of the multilayer zinc alloy platedsteel materials, performing surface adjustment and phosphate treatmentaccording to general specifications of automobile company, andevaluating the uniformity of phosphate.

1. Good: phosphate film was uniformly formed.

2. Poor: phosphate film was not uniformly formed.

TABLE 1 First plating Second plating Third plating layer layer layerPlating Plating Plating Mg Type amount Type amount Type amount weight No. (wt %) (g/m²) (wt %) (g/m²) (wt %) (g/m²) ratio Note 1 Zn 5Zn—Mg(16.5% Mg) 15 — — 0.124 *CE1 2 Zn 5 Zn—Mg(17.0% Mg) 15 — — 0.128CE2 3 Zn 5 Zn—Mg(17.3% Mg) 15 — — 0.130 **IE1 4 Zn 5 Zn—Mg(18.7% Mg) 15— — 0.140 IE2 5 Zn 5 Zn—Mg(20.7% Mg) 15 — — 0.155 IE3 6 Zn 5 Zn—Mg(21.0%Mg) 15 — — 0.158 IE4 7 Zn 5 Zn—Mg(22.0% Mg) 15 — — 0.165 IE5 8 Zn 5Zn—Mg(24.0% Mg) 15 — — 0.180 IE6 9 Zn 5 Zn—Mg(26.0% Mg) 15 — — 0.195 IE710 Zn 5 Zn—Mg(27.3% Mg) 15 — — 0.205 IE8 11 Zn 5 Zn—Mg(29.3% Mg) 15 — —0.220 IE9 12 Zn 5 Zn—Mg(32.0% Mg) 15 — — 0.240 IE10 13 Zn 5 Zn—Mg(32.3%Mg) 15 — — 0.242 CE3 14 Zn 5 Zn—Mg(32.6% Mg) 15 — — 0.245 CE4 15Zn—Mg(5.0% Mg) 5 Zn—Mg(22.3% Mg) 15 — — 0.180 IE11 16 Zn—Mg(8.0% Mg) 5Zn—Mg(21.3% Mg) 15 — — 0.180 IE12 17 Zn—Mg(15.0% Mg) 17 Zn 3 — — 0.128CE5 18 Zn—Mg(17.0% Mg) 17 Zn 3 — — 0.145 IE13 19 Zn—Mg(21.2% Mg) 17 Zn 3— — 0.180 IE14 20 Zn—Mg(25.9% Mg) 17 Zn 3 — — 0.220 IE15 21 Zn—Mg(28.5%Mg) 17 Zn 3 — — 0.242 CE6 22 Zn 5 Zn—Mg(20.8% Mg) 12 Zn 3 0.125 CE7 23Zn 5 Zn—Mg(27.5% Mg) 12 Zn 3 0.165 IE16 24 Zn 10 Zn—Mg(27.5% Mg) 24 Zn 60.165 IE17 25 Zn 5 Zn—Mg (30% Mg) 12 Zn 3 0.180 IE18 26 Zn 10 Zn—Mg (30%Mg) 24 Zn 6 0.180 IE19 27 Zn 5 Zn—Mg(36.6% Mg) 12 Zn 3 0.220 IE20 28 Zn5 Zn—Mg(40.8% Mg) 12 Zn 3 0.245 CE8 *CE: Comparative Example, **IE:Inventive Example

TABLE 2 Phosphate Corrosion Powdering treatment No. Weldabilityresistance resistance properties Note 1 4 1 1 2 *CE1 2 3 1 1 2 CE2 3 2 11 2 **IE1 4 2 1 1 2 IE2 5 2 1 1 2 IE3 6 1 1 1 2 IE4 7 1 1 1 2 IE5 8 1 11 2 IE6 9 1 1 1 2 IE7 10 2 1 1 2 IE8 11 2 1 1 2 IE9 12 2 1 1 2 IE10 13 31 1 2 CE3 14 3 1 1 2 CE4 15 1 1 1 2 IE11 16 1 1 2 2 IE12 17 3 1 3 1 CE518 2 1 3 1 IE13 19 1 1 3 1 IE14 20 2 1 3 1 IE15 21 3 1 3 1 CE6 22 3 1 11 CE7 23 1 1 1 1 IE16 24 2 1 1 1 IE17 25 1 1 1 1 IE18 26 2 1 1 1 IE19 272 1 1 1 IE20 28 3 1 1 1 CE8 *CE: Comparative Example, **IE: InventiveExample

Referring to Table 2, Inventive Examples 1 to 20 satisfying all theconditions proposed in the present disclosure had high corrosionresistance and spot weldability. In addition, it could be found that itis preferable that the Mg weight ratio is within the range of 0.157 to0.20 and the sum of plating amounts of the multiple plating layers bewithin the range of 35 g/m2 or less, so as to further improve spotweldability.

However, since Comparative Examples 1 to 8 had a Mg weight ratio outsidethe range proposed in the present disclosure, the spot weldability ofComparative Samples 1 to 8 was poor.

In addition, referring to Table 2, it could be found that that when thelowest plating layer is a Zn plating layer or a Zn—Mg alloy platinglayer having a Mg content preferably within the range of 7 wt % or less(excluding 0 wt %), platability can be improved, and when the uppermostplating layer is a Zn plating layer or a Zn—Mg alloy plating layerhaving a Mg content preferably within the range of 2 wt % or less(excluding 0 wt %), phosphate treatment properties can be improved.

FIG. 8 is an image showing a weld zone formed by a spot welding in amultilayer zinc alloy plated steel material of Inventive Sample 18.Referring to FIG. 8 , it could be visually checked that the multipleplating layers of the multilayer zinc alloy plated steel material of thepresent disclosure was changed to a single alloy layer after welding,the single alloy layer in the weld zone had a MgZn₂ alloy phase in anarea fraction of 90% or greater, and type B cracks and type C were notformed in the weld zone.

DESCRIPTIONS OF REFERENCE NUMERALS

-   -   100, 200, 300: MULTILAYER ZINC ALLOY PLATED STEEL MATERIALS    -   110, 210, 310: FIRST PLATING LAYERS    -   120, 220, 320: SECOND PLATING LAYERS    -   330: THIRD PLATING LAYER

The invention claimed is:
 1. A multilayer zinc alloy plated steelmaterial comprising: a base steel material having a thickness; andmultiple plating layers formed on the base steel material, wherein aratio of a weight of Mg contained in the multiple plating layers to atotal weight of the multiple plating layers is from 0.13 to 0.24,wherein the multiple plating layers comprise a first plating layerformed on the base steel material and a second plating layer formed onthe first plating layer, the first plating layer comprises a Zn—Mg alloyphase, and the second plating layer comprises a Zn single phase or a Znsingle phase and a Zn—Mg alloy phase and has a Mg content within a rangeof 2 wt % or less.
 2. The multilayer zinc alloy plated steel material ofclaim 1, wherein, when a GDS profile is measured at a thicknesswisecenter portion of each of the multiple plating layers, a Mg content inthe each of the multiple plating layers deviates within a range of ±5%in a width direction of the each of the multiple plating layers which isa direction perpendicular to a rolling direction of the base steelmaterial.
 3. The multilayer zinc alloy plated steel material of claim 1,wherein grains of the multiple plating layers have an average diameterof 100 nm or less (excluding 0 nm).
 4. The multilayer zinc alloy platedsteel material of claim 1, wherein a total plating amount of themultiple plating layers is 40 g/m² or less (excluding 0 g/m²).
 5. Themultilayer zinc alloy plated steel material of claim 1, wherein, whenspot welding is performed on the multilayer zinc alloy plated steelmaterial, the multiple plating layers are configured to be changed to asingle alloy layer in a weld zone, and the single alloy layer comprises:a MgZn₂ alloy phase in an area fraction of 90% or greater (including100%).
 6. The multilayer zinc alloy plated steel material of claim 1,wherein, when spot welding is performed in accordance with SEP 1220-2,an average length of type B cracks is equal to or less than 0.1 timesthe thickness of the base steel material.
 7. The multilayer zinc alloyplated steel material of claim 1, wherein the base steel materialcomprises, by wt %, C: 0.10% to 1.0%, Si: 0.5% to 3%, Mn: 1.0% to 25%,Al: 0.01% to 10%, P: 0.1% or less (excluding 0%), S: 0.01% or less(excluding 0%), and a balance of Fe and inevitable impurities, andwherein the contents of C, Si, Mn, P, and S satisfy Condition 1 below:[C]+[Mn]/20+[Si]/30+2[P]+4[S]≥0.3  Condition 1: where each of [C], [Mn],[Si], [P], and [S] refers to a content (wt %) of a correspondingelement.
 8. The multilayer zinc alloy plated steel material of claim 1,wherein a sum of plating amounts of the first and second plating layersis within a range of 40 g/m² or less (excluding 0 g/m²), and a platingamount of the second plating layer is within a range of 2 g/m² orgreater.
 9. The multilayer zinc alloy plated steel material of claim 1,wherein the ratio is from 0.157 to 0.20.
 10. The multilayer zinc alloyplated steel material of claim 1, wherein a total plating amount of themultiple plating layers is within a range of 10 g/m² to 35 g/m².